1. Field of the Invention:
The present invention relates to a method for neutralization of sulfuric acid containing iron ions as main impurity.
2. Description of the Related Art:
As a method for neutralization of sulfuric acids containing iron ions as main impurity, such as waste sulfuric acids in surface treatments of steel products, in refining processes of non-ferrous metals and in production processes of titanium oxides using sulfuric acid as well as sulfuric acid obtained by distilling or evaporating hydrochloric acid or nitric acid containing iron ions to which sulfuric acid has been added, addition of sodium hydroxide for the neutralization has commonly been adopted. However, such a method is uneconomical because sodium hydroxide is precious and sodium sulfate obtained as by-product is of almost no commercial value.
Use of ammonia gas or ammonium hydroxide has also been conventionally applied in some instances, but commercialization of ammonium sulfate as a by-product is difficult and ammonium sulfate itself is cheap, and therefore, such a method is also not economical. It is possible to use Ca(OH).sub.2 or CaCO.sub.3, which are cheaper than NaOH and NH.sub.4 OH, but disposal of the solid by-product, CaSO.sub.4.2H.sub.2 O (gypsum), is troublesome, so that the use of such a calcium compound is also not desirable.
Accordingly, it is the present status that the factories, in which waste sulfuric acid containing iron ions is formed as by-product, are trying to find a new market of Na.sub.2 SO.sub.4 or to develop an effective use of gypsum (CaSO.sub.4.2H.sub.2 O) However, an effective and economical method for neutralization of waste sulfuric acid containing iron ions has not yet been discovered.
In addition to the neutralization process, there is another way for the recovery and reuse of sulfuric acid solution containing Fe ions, in which the solution is subjected to evaporation under reduced pressure for precipitating FeSO.sub.4.7H.sub.2 O so as to reduce the content of Fe ion in the residual solution. This method is adopted widely, but since it is impossible to obtain pure sulfuric acid containing entirely no Fe ion and a large quantity of energy is consumed thereby for the evaporation and concentration, this method is neither practical nor economical.